What Is Roofing Underlayment and How It Works?

What Is Roofing Underlayment and How It Works?

When you are preparing to roof your home there are many choices as to what types of material and system you should use. There are also different system components that are important aspects of providing a waterproof covering over your home. In this article I would like to discuss the purpose of underlayments and introduce the most common types of underlayments utilized in the market today.

Moisture And Exhaust In Your Home

The first thing to understand is that daily activities in your home produce moist hot hair. This condensation is the worst enemy to building products. Homes are designed to exhaust that moisture into the attic and from the attic to the outside. Since by nature hot air rises there is also a fair amount of the condensation that works its way through the pours of your homes surfaces. Imagine the amount of moisture that is produced by a family of 5 taking daily showers. That is sucked into the attic by the bathroom fans and then the soffit and roof vents remove as much as possible. The trapped hot air continues to work its way organically through the wood roof deck and the shingles. A small portion of that moisture gets trapped between the asphalt shingles and the wood roof deck threatening to cause premature rot and decay. This is where underlayments come into play.

Vapor Barrier

Underlayments are designed to protect the wood roof deck. They keep the trapped moisture from settling on to the sheathing. In most cases there is never going to be enough moisture trapped between the shingles and underlayments to create a flow of water, but a consistent exposure to moisture can do a lot of damage over time. In addition to its primary purpose of being a moisture vapor barrier it underlayments also protect the shingles from the sap in the wood deck. Sap causes shingles to decompose before their normal life rate. One other important function of roof deck protection is it acts as a back up when wind driven rain makes its way under your shingles. Without it the water would go right onto the roof deck, find its way into your attic and then work its way into your home damaging the interior.

Common Types Of Underlayments

The most common type of underlayment is felt. It is designed with a fiberglass matt and covered in asphalt. Felt roof deck protections comes in #15 and #30 strengths. Synthetic underlayments are newer to the market in the last 20 years but have incredible features. For instance they are designed to breathe and aid in getting the moisture out of your attic. It is also very light weight and comes in 10 square rolls. One of the best benefits of synthetic underlayments is the ability to seal around nail penetrations.

Make sure you utilize roofing underlayment when you prepare to re-roof your home!


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